1. Field of the Invention
The present invention relates to a solid-state imaging device which makes use of a charge coupled device (referred to as "CCD", hereinunder) for taking out optical information from a photoelectric conversion element.
2. Description of the Prior Art
Solid-state imaging devices are required to have high resolution equivalent to that of imaging electron tubes used in TV broadcasting. The device, therefore, has a matrix constituted by picture elements arranged in 500 vertical rows and 800 to 1000 horizontal lines arranged on a semiconductor substrate, as well as scanning elements corresponding to the picture elements. Therefore, solid-state imaging devices are produced by making use of MOS large-scale circuit technique capable of forming a high degree of integration. Usually, CCDs or MOS transistors are used as the constituent elements of the solid-state imaging device.
CCD solid-state imaging devices have been developed which employ low-noise CCDs as the charge transfer elements for taking out the signal charges accumulated in photoelectric conversion elements.
FIG. 1 shows the basic arrangement of a CCD solid-state imaging device, which was described in "Study on Overflow CCD Image Sensor" by Oda et al in the transaction of 1981 Conference of All Japan Society of Television Engineers. In this Figure, a reference numeral 1 denotes a photoelectric conversion element constituted by a photodiode, while 2 and 3 denote vertical CCD register (referred to as "vertical CCD", hereinunder) and horizontal CCD register (referred to as "horizontal CCD", hereinunder) for picking up the optical signals accumulated in the photoelectric conversion element group and delivering the same to the output terminal 4 of the signal detecting circuit. Numerals 5-1, 5-2, 6-1 and 6-2 denote generators for generating clock pulses for driving the vertical CCD 2 and the horizontal CCD 3. Although two-phase type clock pulse generators are shown, the invention does not exclude the use of four- or three-phase forms of clocks. Numerals 7-1 and 7-2 designate a transfer gates through which the electric charges stored in the photodiode 1 are delivered to the vertical CCD. The solid imaging device shown in FIG. 1 as it is can be used as a monotone imaging device, and is usable also as a color imaging device having color information in combination with color filters laminated thereon.
FIG. 2 shows sectional views of the CCD solid-state imaging device shown in FIG. 1, taken along the lines x--x' and y--y'. In this Figure, a photodiode of, for example, n-type, 2' denotes a diffusion layer for the burried channel constituting the vertical CCD 2. The diffusion layer 2' is of, for example, the n-type, and is unnecessary in the case of a surface channel. Numerals 2-a and 2-b denote a pair of electrodes constituting the vertical CCD 2, formed by, for example, polycrystalline silicon of the first and second layers, respectively. A numeral 7 designates a transfer gate portion, 8 designates a gate oxide film, e.g., SiO.sub.2, 9 denotes a picture element isolation oxide film (SiO.sub.2), e.g., SiO.sub.2, 10 denotes an isolation oxide film, 11 denotes a light shielding film for preventing any leak of light to the area outside the photodiode region, e.g., a metallic film such as of Al, and 12 denotes a semiconductor substrate of, for example, the p-type. A layer 13 formed on the underside of the electrode 2-a is an impurity layer of, for example, the p-type provided for the purpose of generation of a potential difference between the electrode 2-a and the electrode 2-b. The charges are transferred in the direction of an arrow 14, by virtue of the internal voltage barrier constituted by the impurity layer 13. The films 10 and 11 are omitted from FIG. 2(b).
The solid-state imaging device has various advantages such as reduced size and weight, reduced power consumption, as well as being maintenance-free as compared with the electron tube, owing to its solid-state nature, and it has a promising future as an imaging device. Unfortunately, however, the CCD imaging devices at the present level of technology encounter the following problems which make it difficult to improve the quality of the picture.
Scanning the vertical or row direction is conducted in an interlace manner so that picture element signals of odd-number lines (1, 3, 5, . . . , 2N-1) and picture element signals of even-number lines (2, 4, 6, . . . , 2N) are obtained in the first and the second fields, respectively. Consequently, in the first field of the succeeding frame, the signals of the line which was not read in the immediately preceding field, i.e., the signals of an odd-number line, are read in addition to the new signals. This phenomenon is usually referred to as "after image". The solid-state imaging device is advantageous in that it does not cause any after image by virtue of a high switching speed. Actually, however, a field after image is inevitably generated due to the interlace reading system mentioned above. Also, in a monitor having a high intensity of incidence light, white fringes are formed on the upper and lower sides of an objective image of large incidence light intensity. This phenomenon is usually referred to as "smear". Smear seriously impairs the quality of the picture as is the case of the after image.
Accordingly, it is very important to overcome these problems, in the future use of solid-state imaging devices.